This invention relates to ferritic stainless steels exhibiting markedly improved corrosion resistance to rust and acids.
In general, ferritic stainless steel has been widely used as a corrosion resistance material because it is inexpensive due to the lack of incorporation of nickel (a relatively expense alloying element) and because it exhibits improved resistance to stress-corrosion cracking. However, ferritic stainless steel is inferior to austenitic stainless steel with respect to the resistance to rust, particularly under environments containing chloride ions, and the resistance to acids. This tendency is pronounced on low-Cr ferritic stainless steels containing less than 18% Cr. Generally, nickel is incorporated into such steels to enhance their corrosion resistance. However, if a relatively large amount of nickel is incorporated in a ferritic stainless steel, the resistance to stress-corrosion cracking deteriorates and increase in material cost is unavoidable. It is to be noted that the resistance to stress-corrosion cracking is one of important properties of ferritic stainless steel.
Hitherto, in order to improve the corrosion resistance of ferritic stainless steel it has been proposed to increase the chromium content, to add molybdenum, to reduce the carbon or nitrogen content, or to add stabilizing elements, such as titanium, zirconium and niobium. For example, Japanese Patent Publication No. 5973/1975 discloses a pitting corrosion resistant ferritic stainless steel containing 22-30% Cr and 1.5-3% Mo with the addition of titanium and/or zirconium, and optionally niobium. Japanese Patent Publication No. 13464/1976 discloses a weatherable ferritic stainless steel containing 15-20% Cr and 0.3-1.5% Mo with the content each of carbon and nitrogen being reduced to less than 0.30% and with the addition of zirconium. U.S. Pat. No. 3,807,991 discloses a ferritic stainless steel containing 20.0-35.0% Cr and 0.75-1.20% Mo with the amounts of phosphorous, sulfur, carbon, and nitrogen being restricted to less than given levels, respectively, together with the addition of niobium. All these steels essentially contain molybdenum as well as a stabilizing element such as titanium, zirconium or niobium so as to improve the corrosion resistance. Thus, it is well known in the art that the addition of molybdenum can serve to improve the corrosion resistance, particularly the pitting resistance, of ferritic stainless steel. However, since molybdenum is not only expensive, but also is sharply fluctuating in prices, the molybdenum-containing steel material is not suitable as a material for manufacturing mass-production articles, such as automotive components. In addition, of the above stabilizing elements, titanium and zirconium easily form carbo-nitrides, oxides, etc. thereof with the resulting non-metallic inclusions causing surface defects such as are called "streak flaws" and "white cloudy appearance" when the steel is rolled to a thin sheet. The term "streak flaws" used herein means streak-like defects on the sheet surface caused by the inclusions of carbo-nitrides, etc., which have been extended in the rolling direction during rolling, and the term "white cloudy appearance" means that the metallic luster of the surface has been lost locally or throughout the surface thereof during pickling due to unusual corrosion of said inclusions which have been dispersed in the sheet surface area.
Furthermore, there is another approach to improve the corrosion resistance of ferritic stainless steel. That is, as is already known in the art, the sulfur content is reduced so as to further improve the corrsion resistance, since the presence of sulfur in steel adversely affects the corrosion resistance. For example, "Br. Corros. J." 1972, Vol. 7, March, pp. 90-93 discusses the effect of sulfur and manganese on the pitting corrosion of iron. "Scandinavian Journal of Metallurgy" 5(1976) pp. 16-20 reveals the correlation between the formation of pits and the sulfur content on austenitic stainless steel. In addition, the preprint report to a symposium held by the Japan Academy of Metallurgy (Nov. 15, 1978) pp. 11-15 discloses the effect of sulfur on pitting corrosion and interstitial corrosion. However, all these reports are merely based on the study of the influence of a sulfur content of around 0.003% at the lowest, and as disclosed in FIG. 1 on page 11 of said preprint report it has been concluded that a sulfur content of less than 0.006% does not provide any more substantial effect on the corrosion resistance than a sulfur content of 0.006%. That is, the effect of reducing the sulfur content is flat at a sulfur level of approximately 0.006%. In this connection, it has heretofore been thought in the art that a stainless steel having an extremely low sulfur content such as 0.001% or less is very difficult to put into practical use because of restrictions on the steel refining technology. In fact, the smallest sulfur content exemplified in said U.S. Pat. No. 3,807,991, for example, is only 0.007%, though it suggests that the sulfur content should be kept low.